Microelectronic technologies commonly utilize both thin and thick film microcircuitry on a glass or ceramic substrate; thin films being generally less than 5 microns in thickness (per Mil Std 883-C and the International Society of Hybrid Microelectronics' definition) and thick films being considerably thicker. These films, comprising patterns of resistors, conductors, and sometimes capacitors applied by conventional film processing techniques, may include "add-on" discrete components such as transistors, diodes, etc., which are attached to the conductor and/or resistor portion of the pattern by solder, wire bonds, or other techniques. In the event a resistor film package is desired, these discrete add-on components are not included.
In thin film resistor technology, the resistor/conductor components are often made of vacuum deposited Nichrome, a registered trademark of Driver-Harris Co., comprising 80% by weight of nickel, 20% by weight of chromium. Thin film patterns are generally photo-etched to be long and serpentine in order to provide a sufficient number of squares to achieve the required resistance, whereas thick film patterns are often rectangular, using various resistivity thick films.
When a thin film resistor is deposited, it is often desirable to temperature anneal the resistive element at high temperature in order to adjust its temperature coefficient of resistance (TCR). Typically, the resistive element/substrate is baked for an hour or so at a temperature of approximately 350.degree. C. It is often then desirable to subject the resistive element/substrate to a stabilization bake where the resistive element is heated at a high temperature over a period of several days so that it will retain its desired resistive value when subjected to temperature cycling in an operational circuit. After the stabilization bake, the substrate is scribed into rows and columns so that it may be separated easily into individual die pieces. Package leads are then soldered to conducting "pads" located on the substrate surface, and these package leads may also be secured to the substrate by an epoxy. Other components, such as transistors, diodes, etc., are incorporated as part of the circuit, if desired.
Thick film technology components are fabricated in a multitude of steps beginning initially with the creation of a pattern or "mask" which provides an outline for depositing the resistor/conductor material onto the substrate. Commonly used mask materials include etched metal and emulsion-screens. Emulsion-screens are typically constructed of stainless steel woven mesh utilizing a mesh count of about 40 to 156 per centimeter. The screens are coated with an emulsion which is hardened into a predetermined pattern by exposure to ultraviolet light, with the remaining non-hardened emulsion removed from that portion of the screen where the pattern is to be printed.
After the substrate is cleaned, typically by mechanical scrubbing action or by ultrasonic treatment utilizing deionized water or a proprietary soap, the substrate is blown off with nitrogen and the surface is dehydrated by baking in an oven for a specified time period.
After the substrate has been prepared, the screen is positioned relative to the substrate and the composition to be printed onto the substrate, often referred as an "ink", is applied to the screen. The "active" materials present in the ink composition depend upon the purpose for which the film is intended to be used. The active materials may comprise electrochemical metals or alloys for resistor films, or dielectric materials for insulating films. The screen is positioned a precise distance above the substrate defined as the "snap off distance". A mechanically operated squeegee is moved at a predetermined velocity across the top of the screen at a predetermined angle to push the ink composition through the screen and onto the surface of the substrate. After the squeegee has passed over a portion of the screen, the screen snaps off the surface of the substrate returning to its original position.
The consistency of the ink is important because it must have a sufficiently low viscosity to flow through the screen and then to settle onto the substrate filling in the gaps left by the screen, yet it must be sufficiently viscous to retain its basic shape after the screen has returned to its snap off position. An organic vehicle is normally included as part of the ink composition to provide the desired consistency. The flow characteristics of the ink composition are quite complex and are generally a function of the shear rate of the composition as it is pushed through the screen. The ink is dried and then it is fired in an oven where the organic vehicle and binders are burned off and the ink is bonded to the substrate.
Attachment of the package leads to the electronic circuitry inside the package is accomplished using thermal compression or ultrasonic sealing of aluminum or gold wire leads. The small diameter gold or alumina wire constitute a significant error factor in the overall value of the resistive element, particularly when these leads are applied to small value resistors.
In order to protect the thin or thick film electronic circuitry, as well as to provide a means for heat dissipation, film networks are packaged generally in metal, ceramic or plastic; plastic being the most popular because of its low cost. If, however, the anticipated operating environment of the package is projected to be severe, a hermetic enclosure or coating is required to enclose the electronic component in an inert, dry atmosphere. High temperatures and humidity accelerate chemical processes such as oxidation, corrosion and electrolytic action, which erode the metallic elements, whereas moisture absorption creates mechanical stresses which vary the resistance value of the resistive element.
The bonding materials normally used to provide severe environment hermetic seals are gold-tin eutectic solder or a solder glass such as PbO-ZnO-Pb.sub.2 O.sub.3 ; these solders are for sealing primarily ceramic packages and are undesirable for precision components because of AC coupling effects. Glass is generally not used as a packaging material. This is because easy to seal glasses do not provide complete hermetic sealing while complete hermetic sealing glasses have high sealing temperatures which adversely affect the electronic component.
As noted above, a principal concern in the fabrication of film resistive elements is the maintenance of absolute, as well as relative, values of the patterned resistors. Absolute accuracy is defined as the difference between the actual value of the resistor and the denoted value of the resistor; whereas relative accuracy, which is critical when resistors comprise a voltage divider, is the difference between the actual ratio of the resistor values and the denoted ratio of the resistor values. Modern electronic instruments often require absolute and relative resistor accuracies of several parts per million.
Excellent absolute accuracy can be achieved by laser trimming the resistor pattern. Conventionally, laser trimming is accomplished prior to the final packaging of the resistive element. Packaging, however, whether it be plastic or metal, can often affect the precise resistor values achieved by laser trimming due to the deposition of materials onto the resistive element, as well as the high temperatures utilized in the packaging process.
Other conventional apparatus and methods include those described in U.S. Pat. No. 3,845,443--Fisher, which discloses a glass coated resistive thermometer comprising a resistive element supported on a alumina substrate and covered with a glass precoat. The resistive element and glass precoat are also coated with an alumina top coat which is "welded" to both the glass precoat and the resistive element.
In U.S. Pat. No. 3,926,502--Tanaka, et al, there is disclosed a liquid crystal display cell comprising two glass substrates disposed in a parallel, spaced apart relationship, and hermetically sealed together along their edges by a layer of glass having a melting point of about 450.degree. C., thereby forming a space between the plates for receiving a liquid crystal substance.
In U.S. Pat. No. 3,412,462--Stutzman, et al, there is disclosed a method of making hermetically sealed thin film modules wherein a glass substrate blank is melted onto a metal substrate to form a hermetic glass-to-metal seal.
In U.S. Pat. No. 4,207,604--Bell, et al, there is disclosed a capacitive pressure transducer comprising a pair of disc-shaped members held in an adjacent parallel relationship by a glass frit fired to permanently fuse the two members in said relationship.
Although it has been recognized that glass materials may be fused as "covering" directly to metal and alumina substrates to provide a hermetic seal for electronic circuitry supported on the substrates, it has not been heretofore possible to fuse vitrified glass to a substrate at temperatures sufficiently low to avoid adversely affecting the electronic circuitry. Vitreous glass is a thermoplastic material which melts and flows at the same temperature each time it is thermally processed. Devitrified glass is a thermosetting material which crystalizes by surface nucleation on a time-temperature relationship.
Devitrified glass has been used in substrates because its thermal stability and chemical durability are improved over the original glass. Further, it will fuse at much lower temperatures than vitrified glass. Unfortunately, it is much more permeable to moisture than vitrified glass.
Further, conventional apparatus and methods have not provided for laser trimming of a hermetically sealed resistive element after completion of those process steps which can affect the absolute value of the resistive element.